Studies of age changes in physiologic systems have focused on the renal system of male volunteers of the Baltimore Longitudinal Study of Aging (BLSA). The possible effect of diet, specifically protein intake, on present and future kidney function (creatinine clearance) was examined. Dietary protein has an insignificant effect on simultaneous clearances or on renal function 10-18 years after the diet estimation when studied across the entire age range, and the profound effect of age is taken into account. In separate age groups, there is a small but significant positive correlation of increasing protein and increasing creatinine clearance measured at the same time, but no negative effect of higher protein intake on future clearance. In this normal population, there is no evidence to support the hypothesis that increased protein intake leads to decreased renal function. Water load tests on normal males of the BLSA demonstrated a marked defect in the ability of older subjects to excrete water. This was manifested by lower free water clearance and lower maximum urineflow in the elderly, as well as a change in time course of the response with older subjects reaching their maximum values later than the young. Studies on the glucose tolerance tests in female volunteers of the BLSA and clamp studies on athletes and volunteers from the teaching nursing home populations are reported in the Metabolism Section reports.